Trying to finely tune the tier system for easier evaluation for Gestalt characters.

Besides Tier 5A, the word "often" splits the tier system into two sub-tier of definitely-has, and definitely-has-not.
Tier 5A is "Very Well", while Tier 3AB and Tier 4AB are only "well" in their "area of expertise".
Splitting the definiteness (very often not needed) would make a Tier 5A1 that is stronger than any Tier 1B in its always needed "area of expertise"; and a Tier 5A2 that is never needed in its "area of expertise".

Tier 1A, Tier 1B, Tier 2A, Tier 2B, Tier 3A, Tier 3B, Tier 4A, Tier 4B are split from the definiteness of the word often.
Note that only Tier 1A, Tier 2A, and Tier 3A have game breaking abilities, while Tier 1B, Tier 2B, and Tier 3C are considered overpowered but not game breaking.

The "or" part of the original definition has split the tiers into sub-tiers. The sub-tiers Tier 3C, Tier 4C, Tier 5B, and Tier 5C comes from this splitting.

Commoner does not fit any tier in the original definition. It has no specialization which Tier 6 actually requires having a specialization (area of expertise), though not shining in it!

Tier 1A: Capable of doing absolutely everything, better than classes that specialize in that thing. Capable of solving encounters with a single mechanical ability (game breaker) and little thought from the player. Has world changing powers at high levels. These guys, if played well, can break any campaign can can be very hard to challenge without extreme DM fiat, especially if Tier 3s and below are in the party.

Tier 1B: Capable of doing absolutely everything, but not better than classes that specialize in that thing. Does not have encounter solving mechanical ability.

Tier 2A: Has as much raw power as the Tier 1A classes, but can't pull off nearly as many tricks, and while the class itself is capable of anything, no one build can actually do nearly as much as any Tier 1A classes. Still potentially campaign smashers by using the right abilities, but at the same time are more predictable and cannot always have the right tools for the job. If the Tier 1 classes are countries with 10,000 nuclear weapons in their arsenal, then these guys are countries with 10 nukes. Still dangerous and world shattering, but not quite so many ways. Note that the Tier 2 classes are often less flexible than Tier 3 classes, it's just that their incredible potential power overwhelms their lack of flexibility.

Tier 2B: Has as much raw power as the Tier 1B classes, but cannot put off nearly as many tricks, and while the class itself is capable of anything, no one build can actually do nearly as much as any Tier 1B classes. Not potentially campaign smasher even with the right abilities, and still predictable and not always have the right tools for the job. This sub-tier often has more flexibility than Tier 3 classes; though, Tier 3A may still be more powerful with their one trick pony game breaker.

Tier 3A: Capable of doing one thing quite well, while still being useful when that one thing is inappropriate. Has a mechanical ability that can solve an encounter (one trick pony game breaker), but this is relatively rare and easy to deal with. Challenging such a character takes some though from the DM, but is not too difficult. Will outshine any Tier 5s (except 5A in their specialty unless they have the same specialty) in the party much of the time.

Tier 3B: Capable of doing one thing quite well, while still being useful when that one thing is inappropriate. Does not have any mechanical ability that can solve an encounter. Challenging such a character takes some though from the DM, but is not too difficult. Will outshine any Tier 5s (except 5A in their specialty) in the party much of the time.

Tier 3C: Capable of doing all things, but not as well as classes that specialize in that area. Challenging such a character takes some though from the DM, but is not too difficult. Will outshine any Tier 5s (except 5A in their specialty) in the party much of the time.

Tier 4A: Capable of doing one thing quite well, and still useful when encounters require other areas of expertise. Rarely has any abilities that can outright handle an encounter unless that encounter plays directly to the class's main strength. DMs may sometimes need to work to make sure Tier 4s can contribute to an encounter, as their abilities may sometimes leave them useless. Won't outshine anyone except in specific circumstances play to their strengths. Cannot compete effectively with Tier 1s that are played well.
NOTE: The difference between Tier 4A and Tier 3B is very vague, but the difference is that Tier 4A has at least one area where the character is useless, while Tier 3B is capable in all areas, just not as well as Tier 3C outside of Tier 3B's specialization. Usually a Gestalt of 4A + 4C = 3B, since 4C would close the weakness of 4A.

Tier 4B: Capable of doing one thing quite well, but useless when encounters require other areas of expertise. Rarely has any abilities that can outright handle an encounter unless that encounter plays directly to the class's main strength. DMs may sometimes need to work to make sure Tier 4s can contribute to an encounter, as their abilities may sometimes leave them useless. Won't outshine anyone except in specific circumstances play to their strengths. Cannot compete effectively with Tier 1s that are played well.

Tier 4C: Capable of doing many things to a reasonable degree of competence without truly shining. Rarely has any abilities that can outright handle an encounter unless that encounter plays directly to the class's main strength. DMs may sometimes need to work to make sure Tier 4s can contribute to an encounter, as their abilities may sometimes leave them useless. Won't outshine anyone except Tier 6s. Cannot compete effectively with Tier 1s that are played well.

Tier 5A: In some cases, can do only do one thing "very well", better than even Tier 3B, but that one thing is very often not needed. Has trouble shining in any encounter unless the rest of the party is weak in that situation and the encounter matches their strengths. DMs may have to work to avoid the player feeling that their character is worthless unless the entire party is Tier 4 and below. Characters in this tier will often feel like one trick ponies if they do well or just feel like they have no tricks at all if they build the class poorly.

Tier 5B: Capable of doing only one thing, and not necessarily all that well, and in many times of encounters the character cannot contribute. Has trouble shining in any encounter unless the rest of the party is weak in that situation and the encounter matches their strengths. DMs may have to work to avoid the player feeling that their character is worthless unless the entire party is Tier 4 and below. Characters in this tier will often feel like one trick ponies if they do well or just feel like they have no tricks at all if they build the class poorly.

Tier 5C: So unfocused that they have trouble mastering anything, and in many times of encounters the character cannot contribute. Has trouble shining in any encounter. DMs may have to work to avoid the player feeling that their character is worthless unless the entire party is Tier 4 and below. These characters are not even one trick pony like 5A and 5B.

Tier 6A: Not even capable of shining in their own area of expertise. DMs will need to work hard to make encounters that this sort of character can contribute in their mechanical abilities. Will often feel worthless unless the character is seriously power-gamed beyond belief, and even then won't be terribly impressive. Needs to fight enemies of lower than normal CR. Class is often completely synergized or almost no ability of merit. Avoid allowing PCs to play these characters.

Tier 6B: Commoner since it has no area of expertise, the Commoner class is designed to be the weakest class of all the classes. Using any reasonable judgement, Infested Chicken is not broken.